Carbon and nitrogen ratios of aquatic and terrestrial prey for freshwater fishes

Ecological subsidies are materials and energy that cross a boundary between two ecosystems. An example is terrestrial leaf litter that falls into a stream to ultimately become the basis of the stream food web. Terrestrial arthropods that fall into streams are often an important food resource for fishes, but variation in the nutritional quality of aquatic versus terrestrial items has not been fully assessed. Carbon and nitrogen ratios in invertebrates roughly reflect relative amounts of structural chitin and since chitin is mostly indigestible to fishes, it can be an indicator of food quality, where a greater amount of chitin (or greater C:N) indicates lower quality. To better understand potential differences in the quality of terrestrial versus aquatic arthropods, we sampled terrestrial and aquatic arthropods during winter, spring, and summer, and measured their molar C:N values. We tested arthropod C:N values for origin (terrestrial vs. aquatic), taxonomy (order level), and time of year (winter, spring, and summer). We did not detect significant differences in any of these comparisons. The average molar C:N (± 1 SD) for aquatic arthropods (n = 81 samples) was 5.0 ± 0.6 and ranged from 4.7 to 5.6 and that for terrestrial arthropods (n = 42 samples) was 5.1 ± 0.7 and ranged from 4.4 to 5.6. Molar C:N values were not different among aquatic and terrestrial arthropods (df = 74.3, t = −0.995, p = 0.418) and did not differ across arthropod taxa (F7,50 = 1.9, p = 0.087). Any trends in molar C:N variability in arthropods, whether they be aquatic or terrestrial, are probably due to relative amounts of structural chitin. Since chitin is largely undigested in fish diets, molar C:N might be important in considering the true benefits of terrestrial subsidies on fish life histories.